27-million-year-old fossil of newly-discovered toothed whale species provides clues about evolution of high-frequency hearing.
Echovenator sandersi produces sound that bounces off prey, creating echoes; the whale’s inner ear receives the sound waves. Image credit: A. Gennari.
In a study published in the journal Current Biology, a team of paleontologists describes Echovenator sandersi, an ancient relative of modern dolphins, and its ability to hear frequencies well above the range of hearing in humans.
“Our study suggests that high-frequency hearing may have preceded the emergence of echolocation,” said lead author Dr. Morgan Churchill, from New York Institute of Technology.
“Previous studies have looked at hearing in whales but our study incorporates data from an animal with a very complete skull,” he added.
“The data we gathered enabled us to conclude that it could hear at very high frequencies, and we can also say with a great degree of certainty where it fits in the tree of life for whales.”
Echovenator sandersi flourished during the Oligocene epoch, about 27 million years ago.
It was a small, toothed whale that probably used its remarkable sense of hearing to find and pursue fish with echoes only.
“This would allow it to hunt at night, but more importantly, it could hunt at great depths in darkness, or in very sediment-choked environments,” said co-author Dr. Jonathan Geisler, also from New York Institute of Technology.
To learn more about Echovenator sandersi, the scientists studied the whale’s skull discovered in South Carolina 2001.
They CT scanned Echovenator sandersi’s ear and compared it to those of two hippos and 23 fossil and living whales. The analyses uncovered many features found today in dolphins, which can hear at ultrasonic frequencies.
The anatomy of the ancient whale’s ear suggests that most of the specializations associated with high frequency hearing evolved about 27 million years ago (about the same time as echolocation) and that traits associated with this ability actually predate the emergence of toothed whales.
It also suggests that the evolutionary ancestors of toothed whales could hear at higher frequencies than their relatives on land.
“The inner ear of Echovenator sandersi is surprisingly similar to that of modern whales,” Dr. Churchill said.
In fact, only one trait of the ancient whale’s ear was more similar to primitive whales than to modern whales, suggesting a very rapid evolution of hearing abilities in early whales.
“About 60 million years ago, the semiaquatic ancestor of whales had a limited ability to hear high frequencies,” the paleontologists said.
“Some degree of high-frequency hearing evolved before echolocation and then became even more specialized in modern toothed whales. Baleen whales, which do not echolocate and are specialized to hear low frequency sound, lost some of these initial specializations for hearing high frequency sound.”
“Echovenator sandersi is remarkable in other ways too,” Dr. Churchill said.
“For instance, the ancient whale was remarkably small compared to its ancestors, suggesting a drastic change in body size early in toothed whale evolution that most likely influenced a range of variables, from brain size to ecology.”
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Morgan Churchill et al. The Origin of High-Frequency Hearing in Whales. Current Biology, published online August 4, 2016; doi: 10.1016/j.cub.2016.06.004
